Achieving single mode emission by introducing perturbations at prescribed positions along the length of a device is known, see EP 1 214 763 (Trinity College Dublin) the contents of which are incorporated herein by reference. So called “slotted lasers”, which achieve single longitudinal mode emission by means of optical feedback resulting from the etching of slot features along the laser cavity, are also disclosed in Irish Patent No S82521 (National University of Ireland, Cork) and S83622 (Eblana Photonics Ltd.) the contents of which are also incorporated herein by reference.
In general terms, the perturbations may be caused by any index altering means which modifies the refractive index profile of the waveguide to an appropriate degree to manipulate optical feedback and hence the spectral content of the device.
The term ‘slot length’ (designated Lslot in the prior art drawing FIG. 1) as used herein refers to the distance between the longitudinal slot faces in the device material, ie ‘slot length’ is measured along the direction of light propagation, d. FIGS. 1 and 2 illustrate a typical prior art slotted laser 1 having a single rectangular slot 6. Typically such a device comprises waveguiding layers 2 (containing for example a multiple quantum well structure) covered by an upper cladding layer 4. Primary optical feedback means are provided in the form of a cleaved facet 8 at either end of the device. The distance between the facets and the effective index along the cavity determines the wavelength's of the Fabry Perot modes. The upper cladding layer 4 forms a ridge 3 having a cap layer 5. The slot features in such known devices are formed by etching a rectangular slot 6 in the ridge waveguide 3, resulting in two longitudinal interfaces 7 that are perpendicular to the direction of light propagation, d, within the device.
The mechanism whereby slotted lasers achieve their single mode performance has previously been described (e.g. Irish Patent Number S83622, the entire contents of which are incorporated herein by reference).
While the description of the present application which follows refers primarily to the case where the perturbations are defined by slots etched along the device it will be appreciated by a person skilled in the art that the teaching of the application is equally applicable to other forms of suitable perturbations.
The present application considers the scattering and reflection processes which occur in these devices. As discussed in greater detail below, the Applicants have realized that in prior art lasing devices scattering and reflection processes interfere destructively.